This invention relates to a method for completing wells and especially to a method for preventing migration of formation fluids from one formation to another through the use of special cementing techniques.
In the completion of wells, it has long been conventional to pump cement down through the casing to flow back up through the casing-borehole annulus to a selected height after which it is permitted to set. This is usually referred to as a "primary cement job". One important goal sought to be achieved in a successful primary job is to create a permanent, fluid-tight seal against vertical fluid communication along the casing-borehole annulus, both before and after perforating. Because the borehole penetrates natural barriers to vertical flow such as shale breaks or otherwise impermeable strata between permeable zones, the cement sheath in the annulus must act in place of those barriers when a differential presssure exists across them. Pressure differential may be induced across barriers either by increasing or decreasing pore pressure relative to that of adjacent zones, or such differentials may exist naturally between normal and geopressure strata. For optimum performance of cased hole completions, interzonal communication between differentially pressured strata is undesirable. Interzonal flow can cause the loss of valuable hydrocarbons, the failure of stimulation treatments, and other problems. Assuming that the cement is impermeable, there are two possible paths for flow between zones or formations. One such path that could develop to allow fluids to move vertically is along the casing-cement interface. Another possible and more probable path is the cement-formation interface. Such flow from one zone formation to another is commonly called "migration."
A recent study has indicated that the failure of a so-called "cement-formation" bond may be a major cause for unsuccessful primary cementing jobs. See "Field Measurements of Annular Pressure and Temperature During Primary Cementing" by C. E. Cooke, Jr., M. P. Kluck and R. Medrano, Society Petroleum Engineers Paper No. 11206, presented at the 57th Annual Fall Technical Conference, New Orleans, La., Sept. 26-29, 1982. Thus, it is indicated that at a particular depth in a well, the hydrostatic pressure exerted by the cement against the formation decreases as the cement cures. Cement, like drilling muds, has thixotropic properties so that after it stops flowing, it develops gel strength so that the column of cement tends to become self supporting, due in part to its frictional engagement with the borehole wall. As a result, when the cement undergoes curing and concurrent shrinkage, the pressure exerted by the cement against the face of the borehole decreases to a point such that the cement does not have sufficient contact with the wall to form a seal therewith. Fluid from a formation can flow or migrate upwardly, for example, through a micro-annulus between the borehole wall and the cement to an upper lower pressure formation or even to the surface of the well. As a result, there can be interzonal fluid communication along the micro-annulus in the direction of a lower pressure zone, be it above or below the zone of interest.
It is therefore an object of this invention to provide a method for isolating one formation from another in a manner such that the shrinkage of cement does not reduce the seal load on the formation to such an extent that migration can occur.